The present invention relates to a microlens array sheet having a plurality of semispheroid microlenses cut from a spheroid material or semispherical microlenses cut from a spherical material, and a method of producing such a microlens array sheet. Particularly, this invention relates to a microlens array sheet useful for screens, for example, in rear-projection televisions, and a method of producing such a microlens array sheet.
Displays are classified into a direct-view type for direct-viewing images displayed on a cell assembly and a projection type for viewing images projected onto a screen by front or rear projection. The latter can be manufactured in a large type at low cost and are gradually popular, especially, in North America and China.
A rear-projection display is a projection type. This type of display mostly employs a lenticular lens array sheet for its screen. The lenticular lens array sheet is, usually, produced by extrusion or injection molding, or press roll with a photocurable resin.
However, the lenticular lens array sheet has a problem in that it gives a wider angle of field only either a horizontal or vertical direction.
In order to solve such a problem, Japanese Un-examined Patent Publication No. 2001-305315 proposes a screen equipped with a light-shielding layer having a microlens array sheet with a lens function.
The microlens array sheet has microlens aligned on a flat base, as concave and convex sections of the base. A screen equipped with such a microlens array sheet is applicable to displays and expected to be popular in the near future.
Japanese Un-examined Patent Publication No. 2001-305315 discloses a microlens array sheet. Formed on a surface of a lens substrate is a plurality of microlens. Formed on the other surface of the substrate is a light-shielding layer having a circular or square light-emitting section.
The Un-examined Patent Publication gives one requirement (Sr≧2t×tan ⊖+R) to the microlens array sheet, in which “Sr” is the size of the microlens, “t” is the thickness of the lens substrate, “⊖”, equal to sin−1 (1/n), is the maximum incident angle, “n” is a refractive index of the lens substrate, and “R” is the diameter of the light-emitting section.
However, a microlens array sheet designed according to the expression shown above sometimes causes optical vignetting, a phenomenon in which transmissivity (the amount of an outgoing beam/the amount of an incoming beam) is decreased when a part of an incoming beam incident along a correct optical path is blocked due to spherical aberration. This is because the above expression does not include a focal point of each microlens.
According to the expression, the maximum incident angle “⊖” depends the refractive index “n” of the lens substrate. Thus, an outgoing beam emitted from the light-emitting section originated from an incoming beam having the maximum incident angle “⊖” sometimes causes optical vignetting to other incoming beams.
Wider apertures for the light-shielding layer to avoid optical vignetting decreases a ratio of the area of black on the light-shielding layer to the total area of a light diffusion plate, thus causing lower contrast for projected images.
There is thus a demand for a microlens array sheet and a production method thereof that achieve both restriction of optical vignetting and enhancement of contrast inconsistent with each other.